Subject: Re: [PATCH RFC tip/core/rcu 05/11] rcu: add comment saying why
 DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT.
From: Steven Rostedt <rostedt@goodmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org, josh@joshtriplett.org,
        niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org,
        Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com,
        darren@dvhart.com
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Date: Wed, 23 Feb 2011 09:11:43 -0500
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On Wed, 2011-02-23 at 08:59 -0500, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@goodmis.org) wrote:
> > On Tue, 2011-02-22 at 17:39 -0800, Paul E. McKenney wrote:
> > > The build will break if you change the Kconfig to allow
> > > DEBUG_OBJECTS_RCU_HEAD and !PREEMPT, so document the reasoning
> > > near where the breakage would occur.
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > ---
> > >  kernel/rcupdate.c |    5 +++++
> > >  1 files changed, 5 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> > > index afd21d1..f3240e9 100644
> > > --- a/kernel/rcupdate.c
> > > +++ b/kernel/rcupdate.c
> > > @@ -214,6 +214,11 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
> > >  		 * Ensure that queued callbacks are all executed.
> > >  		 * If we detect that we are nested in a RCU read-side critical
> > >  		 * section, we should simply fail, otherwise we would deadlock.
> > > +		 * Note that the machinery to reliably determine whether
> > > +		 * or not we are in an RCU read-side critical section
> > > +		 * exists only in the preemptible RCU implementations
> > > +		 * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why
> > > +		 * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT.
> > >  		 */
> > 
> > Shouldn't this comment also be in the kconfig where
> > DEBUG_OBJECTS_RCU_HEAD is defined?
> 
> hrm, but this is a "rcuhead_fixup_init" : it does not need to always

Looks like it was in rcuhead_fixup_free to me.

> succeed. It's just that when it is safe to recover from an error
> situation, it does it. We could do:
> 
> #ifdef CONFIG_PREEMPT
>                 /*
>                  * Ensure that queued callbacks are all executed.
>                  * If we detect that we are nested in a RCU read-side
>                  * critical
>                  * section, we should simply fail, otherwise we would
>                  * deadlock.
>                  */
>                 if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
>                     irqs_disabled()) {
>                         WARN_ON(1);
>                         return 0;
>                 }
>                 rcu_barrier();
>                 rcu_barrier_sched();
>                 rcu_barrier_bh();
>                 debug_object_init(head, &rcuhead_debug_descr);
>                 return 1;
> #else
> 		return 0;
> #endif
> 
> instead, no ?

The point is that this entire block of code is wrapped in #ifdef
CONFIG_DEBUG_OBJECTS_RCU_HEAD, and that config depends on PREEMPT. Thus
you will never have a case where #ifdef CONFIG_PREEMPT is false.

-- Steve


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